I don't know what a MK2? MK1? dash looks like since you're posting in the MK3 sub-forum but the MK3's temp gauge doesn't reflect an accurate reading.Heater core? My engine temp is not high enough, odometer test shows not more than 150F*. Is the heater core related to engine temp???
Could your t-stat be stuck open? It's possible. With fall/winter upon you in the far north, it might be worth replacing it anyways.
Replace the t-stat, flush the coolant, flush the heater core, new coolant and a new t-stat. At least you'd know for sure that you ruled everything out.